The Tacoma Police Department admitted Wednesday to using a cell site simulator, also sometimes called a Stingray, to find suspects of felony-level crimes since 2009. Police said they only use it with permission from a judge.

The device, small enough to fit in a car, can be driven to a neighborhood to help police find a suspect by tracking the signal of the cellphone he carries. But in the process, the Stingray also sweeps up data from everyone else nearby, technology experts say.

Police, who sought permission last year from the Tacoma City Council to update the equipment, told the council it was to be used to find and prevent improvised explosive devices.

Assistant Police Chief Kathy McAlpine said in a statement Wednesday that the police department has never used its cell site simulator to find an improvised explosive device.

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“The equipment is used to locate a specific cellular device, in accordance with applicable state and federal laws to further investigative public safety matters,” McAlpine wrote. “It is used in felony level crimes to locate suspects wanted for crimes such as homicide, rape, robbery, kidnapping, and narcotics trafficking.”

Police also use it to find missing or endangered people. In one instance, records show, the police used it to try to find a stolen city laptop.

McAlpine said in emails that police deploy the device only when judges sign what is called a “pen/trap/trace authorization.” Traditionally this order has allowed police to then approach a cellphone provider and pay the company for the suspect’s location and call logs.

In response to a question asking whether judges know the Police Department will be using a cell site simulator when they give approval for police access to cellphone records, McAlpine wrote only: “We obtain a search warrant under the authority of (state law).”

McAlpine said the police do not collect the content of voice calls, texts or data transfers. She also said: “We don’t collect data, any further explanation is a violation of the NDA,” making reference to the non-disclosure agreement the city has with the FBI.

But the device does collect the unique identifiers of all cellphones in the area, said Christopher Soghoian, principal technologist with the American Civil Liberties Union in Washington, D.C. Police would need to analyze that data to pinpoint a specific person.

Soghoian said he inferred from McAlpine’s statement that police are not retaining the information. He says the device still receives the information.

“What that means is they are not saving data. The Stingray device tells you the serial number of the device of everyone nearby,” Soghoian said. “There is no way to locate a particular phone without also identifying all of the other phones.”

The Stingray also can sweep metadata — who you call and text, when you contact them and for how long you talk — from every device within range, Soghoian said.

The Stingray fools cellphones into connecting with it by pretending to be a cell tower with a strong signal, exploiting a flaw in cellphone security.

Tacoma has helped several other agencies search for targets since it started using the device. Records show it has used the device 179 times since 2009 — mostly for drug cases.

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The device has been deployed 129 times for Tacoma Police Department cases, 14 times for the Pierce County Sheriff’s Department, and a few times for several other agencies, including Lakewood, the Drug Enforcement Agency in both Seattle and Tacoma, Kitsap County, King County, Immigration and Customs Enforcement and various other initiatives or task forces.

Ronald Culpepper, the presiding judge of Pierce County Superior Court, said Wednesday that this type of surveillance device has not been mentioned in warrant applications that he has seen.

Other judges were surprised when they heard TPD was using this technology, he said. “People had never heard of it.” He expects judges to discuss the legal implications of Stingray use in the next couple of weeks.

The head of the Pierce County Department of Assigned Counsel, Michael Kawamura, said he is researching case law on the topic.

“There’s predictably not an abundance of case law because it’s hard to contest something that you didn’t know was happening,” Kawamura said.

Mayor Marilyn Strickland defended the police’s acquisition of the device Tuesday, saying, “If our law enforcement need access to information to prevent crime or keep us safe, that’s a legitimate use of the technology.”